A work machine such as, for example, an off-highway truck, a loader, a motor grader, or any other work machine known in the art may include a multi-speed bidirectional transmission that has one or more hydraulically actuated friction clutches. These clutches may be selectively engaged to produce a predetermined output ratio of the transmission in either a forward or reverse direction. Clutches of this type may include actuators fluidly connected to a main relief valve, which is configured to control a pressure of a hydraulic fluid supplied to the actuators by allowing a flow of fluid to bypass the actuators at a predetermined pressure.
One such transmission is described in U.S. Pat. No. 5,941,358 (the '358 patent) to Hosseini et al. The '358 patent teaches a power distribution system having a pump, a plurality of fluid-actuated clutches, and a plurality of proportional control valves configured to selectively direct pressurized fluid from the pump to the clutches. The '358 patent also teaches a pressure relief valve being disposed upstream of the plurality of proportional control valves for controlling a fluid pressure supplied to the proportional control valves. When an input mechanism of the '358 patent is in a neutral position, all of the flow from the pump is bypassed across the pressure relief valve to a reservoir at a predetermined pressure. When the input mechanism is in an engaged gear position, fluid is initially allowed to flow through one or more of the control valves to actuate one or more of the clutches. Once the appropriate clutches are actuated, all of the flow from the pump is again bypassed across the pressure relief valve at the predetermine pressure.
Although the pressure relief valve of the '358 patent may sufficiently control the pressure of the fluid supplied to the control valves, its location relative to the control valves may be problematic. In particular, because the pressure relief valve bypasses fluid before the fluid flows past the control valves, the control valves may be exposed to stagnant fluid for a majority of the operational time of the transmission. This stagnant fluid may allow for the deposit of dirt and debris in the immediate vicinity of the control valves, which may ultimately result in malfunction or premature failure of the control valves.
The disclosed transmission is directed to overcoming one or more of the problems set forth above.